Method of Making A Flexographic Printing Plate

ABSTRACT

A method of producing a flexographic printing plate using a continuous liquid interphase is provided herein. This method allows for significantly reduced production times and fewer preparation steps compared to standard non-continuous techniques and results in less waste than typical methods for preparing flexographic printing plates. The printing plate provided by using continuous liquid interphase production results in a printing plate with desirable elastomeric elongation, desirable hardness, plate thickness in the range of 0.030 inches to 0.250 inches, and comprises printing dots with desirable characteristics.

FIELD OF THE INVENTION

The present invention relates generally to an improved method of makingflexographic printing plates.

BACKGROUND OF THE INVENTION

Flexography is a method of printing that is commonly used forhigh-volume runs. Flexographic printing plates are employed for printingon a variety of substrates such as paper, paperboard stock, corrugatedboard, films, foils and laminates. Newspapers and grocery bags areprominent examples. Coarse surfaces and stretch films can beeconomically printed only by means of flexography.

Flexographic printing plates are relief plates with image elementsraised above open areas. Generally, the plate is somewhat soft, andflexible enough to wrap around a printing cylinder, and durable enoughto print over a million copies. Such plates offer a number of advantagesto the printer, based chiefly on their durability and the ease withwhich they can be made. A typical flexographic printing plate asdelivered by its manufacturer is a multilayered article made of, inorder, a backing or support layer; one or more unexposed photocurablelayers; optionally a protective layer or slip film; and often, aprotective cover sheet.

The support (or backing) layer lends support to the plate. The supportlayer can be formed from a transparent or opaque material such as paper,cellulose film, plastic, or metal. Preferred materials include sheetsmade from synthetic polymeric materials such as polyesters, polystyrene,polyolefins, polyamides, and the like. One widely used support layer isa flexible film of polyethylene terephthalate.

The photocurable layer(s) can include any of the known polymers,monomers, initiators, reactive and/or non-reactive diluents, fillers,and dyes. As used herein, the term “photocurable” refers to acomposition which undergoes polymerization, cross-linking, or any othercuring or hardening reaction in response to actinic radiation with theresult that the unexposed portions of the material can be selectivelyseparated and removed from the exposed (cured) portions to form athree-dimensional relief pattern of cured material. Exemplaryphotocurable materials are disclosed in European Patent Application Nos.0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No.1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No.3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos.4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S.Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz,et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No.4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., thesubject matter of each of which is herein incorporated by reference inits entirety.

Photocurable materials generally cross-link (cure) and harden throughradical polymerization in at least some actinic wavelength region. Asused herein, “actinic radiation” refers to radiation that is capable ofpolymerizing, crosslinking or curing the photocurable layer. Actinicradiation includes, for example, amplified (e.g., laser) andnon-amplified light, particularly in the ultraviolet (UV) and violetwavelength regions.

The slip film is a thin layer, which protects the photopolymer from dustand increases its ease of handling. In a conventional (“analog”) platemaking process, the slip film is transparent to UV light, and theprinter peels the cover sheet off the printing plate blank, and places anegative on top of the slip film layer. The plate and negative are thensubjected to flood-exposure by UV light through the negative. The areasexposed to the light cure, or harden, and the unexposed areas areremoved (developed) to create the relief image on the printing plate.

In a “digital” or “direct to plate” process, a laser is guided by animage stored in an electronic data file, and is used to create an insitu negative in a digital (i.e., laser ablatable) masking layer, whichis generally a slip film which has been modified to include a radiationopaque material. Portions of the laser ablatable layer are then ablatedby exposing the masking layer to laser radiation at a selectedwavelength and power of the laser. Examples of laser ablatable layersare disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al.,and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter ofeach of which is herein incorporated by reference in its entirety.

Processing steps for forming flexographic printing plates with reliefimage printing elements typically include the following:

1) Image generation, which may be mask ablation for digital “computer toplate” printing plates or negative production for conventional analogplates;

2) Back exposure to create a floor layer in the photocurable layer andestablish the depth of relief;

3) Face exposure through the mask (or negative) to selectively crosslinkand cure portions of the photocurable layer not covered by the mask,thereby creating the relief image;

4) Development to remove unexposed photopolymer by solvent (includingwater) or thermal development; and

5) If necessary, post exposure and detackification.

Removable coversheets are also preferably provided to protect thephotocurable printing element from damage during transport and handling.Prior to processing the printing elements, the coversheet is removed andthe photosensitive surface is exposed to actinic radiation in animagewise fashion. Upon imagewise exposure to actinic radiation,polymerization, and hence, insolubilization of the photopolymerizablelayer occurs in the exposed areas. Treatment with a suitable developersolvent (or alternatively, thermal development) removes the unexposedareas of the photopolymerizable layer, leaving behind a printing reliefthat can be used for flexographic printing.

As used herein “back exposure” refers to a blanket exposure to actinicradiation of the photopolymerizable layer on the side opposite thatwhich does, or ultimately will, bear the relief. This step is typicallyaccomplished through a transparent support layer and is used to create ashallow layer of photocured material, i.e., the “floor,” on the supportside of the photocurable layer. The purpose of the floor is generally tosensitize the photocurable layer and to establish the depth of relief.

Following the brief back exposure step (i.e., brief as compared to theimagewise exposure step which follows), an imagewise exposure isperformed utilizing a digitally-imaged mask or a photographic negativemask, which is in contact with the photocurable layer and through whichactinic radiation is directed.

After imaging, the photosensitive printing element is developed toremove the unpolymerized portions of the layer of photocurable materialand reveal the crosslinked relief image in the cured photosensitiveprinting element. Typical methods of development include washing withvarious solvents or water, often with a brush. Other possibilities fordevelopment include the use of an air knife or thermal development,which typically uses heat plus a blotting material. The resultingsurface has a relief pattern, which typically comprises a plurality ofdots that reproduces the image to be printed. After the relief image isdeveloped, the resulting relief image printing element may be mounted ona press and printing commenced. In addition, if necessary, after thedevelopment step, the relief image printing element may be post exposedand/or detackified as is generally well known in the art.

The shape of the dots and the depth of the relief, among other factors,affect the quality of the printed image. It is also very difficult toprint small graphic elements such as fine dots, lines and text usingflexographic printing plates.

In addition, maintaining small dots on flexographic plates can be verydifficult due to the nature of the platemaking process. In digitalplatemaking processes that use a UV-opaque mask layer, the combinationof the mask and UV exposure produces relief dots that have a generallyconical shape. The smallest of these dots are prone to being removedduring processing, which means no ink is transferred to these areasduring printing (i.e., the dot is not “held” on plate and/or on press).Alternatively, if the dots survive processing they are susceptible todamage on press. For example small dots can fold over and/or partiallybreak off during printing, causing either excess ink or no ink to betransferred.

As described in U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub.No. 2011/0079158 to Recchia et al., the subject matter of each of whichis herein incorporated by reference in its entirety, a particular set ofgeometric characteristics can define a flexographic printing plate dotshape that yields superior printing performance, including but notlimited to (1) planarity of the dot surface; (2) shoulder angle of thedot; (3) depth of relief between the dots; and (4) sharpness of the edgeat the point where the dot top transitions to the dot shoulder.

Flexographic printing elements can additionally be made from liquidphotopolymer resins and have the advantage that the uncured resin can bereclaimed from the non-image areas of the printing elements and used tomake additional printing plates. Liquid photopolymer resins have afurther advantage as compared to sheet polymer in terms of flexibility,which enables the production of any required plate gauge simply bychanging the machine settings. The plates are typically formed byplacing a layer of liquid photopolymerizable resin on a glass plate butseparated from the glass plate by a substrate and/or a coverfilm.Actinic light, such as UV light, is directed against the resin layerthrough a negative. The result is that the liquid resin is selectivelycross-linked and cured to form a printing image surface that mirrors theimage on the negative. Upon exposure to actinic radiation, the liquidphotopolymer resin polymerizes and changes from a liquid state to asolid state to form the raised relief image. After the process iscomplete, non-crosslinked liquid resin can be recovered (i.e.,reclaimed) from the printing plates and recycled in the process to makeadditional plates.

Various processes have been developed for producing printing plates fromliquid photopolymer resins as described, for example, in U.S. Pat. No.5,213,949 to Kojima et al., U.S. Pat. No. 5,813,342 to Strong et al.,U.S. Pat. Pub. No. 2008/0107908 to Long et al., and in U.S. Pat. No.3,597,080 to Gush, the subject matter of each of which is hereinincorporated by reference in its entirety.

Typical steps in the liquid platemaking process include:

(1) casting and exposure;

(2) reclamation;

(3) washout;

(4) post exposure;

(5) drying; and

(6) detackification.

In the casting and exposure step, a photographic negative is placed on aglass platen and a coverfilm is placed on the negative in an exposureunit. All of the air is then removed by vacuum so that any wrinkling ofthe negative or coverfilm can be eliminated. Thereafter, a layer ofliquid photopolymer and a backing sheet (i.e., a thin layer of polyesteror polyethylene terephthalate) are applied on top of the coverfilm andnegative. The backing sheet may be coated on one side to bond with theliquid photopolymer and to serve as the back of the plate afterexposure. Then upper and/or lower sources of actinic radiation (i.e., UVlights) are used to expose the photopolymer to actinic radiation tocrosslink and cure the liquid photopolymer layer in the areas notcovered by the negative. The top sources of actinic radiation are usedto create the floor layer of the printing plate (i.e., back exposure)while the bottom sources of actinic radiation are used to face exposethe photopolymer to actinic radiation through the negative to create therelief image.

After the exposure is complete, the printing plate is removed from theexposure unit and the photopolymer that was not exposed to actinicradiation (i.e., the photopolymer covered by the negative) is reclaimedfor further use. In liquid platemaking, resin recovery is an importantfactor relating to the production of photopolymerizable resin printingplates because the resins used to produce the plates are relativelyexpensive. In all areas not exposed to UV radiation, the resin remainsliquid after exposure and can then be reclaimed. In a typical process,the uncured resin is physically removed from the plate in a process stepso that the uncured resin can be reused in making additional plates.This “reclamation” step typically involves squeegeeing, vacuuming orotherwise removing liquid photopolymer remaining on the surface of theprinting plate.

Stereolithography is yet another conventional process for providingflexographic printing plates, which is an additive layering process thatis very time consuming. Each layer of photopolymer is cured, lifted,back filled with more resin, cured again, and the process is repeatedover and over until the required thickness and plate properties areachieved. The process of creating flexographic printing plates usingstereolithography can take anywhere from hours to more than a day.

Thus, it would be desirable to provide an improved method of makingflexographic printing plates, which involves fewer process steps, isless time consuming, creates less waste, and reliably provides printingplates comprising printing dots with desirable characteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a flexographicprinting plate using a continuous liquid interphase 3D productionmethod.

It is still another object of the present invention to streamline theprocess of making a flexographic printing plate.

It is still another object of the present invention to provide animproved method of creating a flexographic printing plate havingtailored printing dots in terms of edge definition, shoulder angleand/or print surface.

It is another object of the present invention to provide a method oftailoring or modifying the shape of printing dots on a flexographicprinting plate made using a continuous liquid interphase 3D method foroptimal printing on various substrates.

It is still another object of the present invention to provide aphotocurable composition for use in the production of flexographicprinting plates using a continuous liquid interphase 3D productionmethod.

It is still another object of the current invention to create little tono waste using a continuous liquid interphase method of producingflexographic printing plates.

To that end, in one embodiment, a continuous liquid interphase method ofmaking a flexographic printing plate is provided, comprising:

-   -   a) providing a photocurable composition in a reservoir, wherein        the photocurable composition comprises:        -   i) a binder resin;        -   ii) monomers;        -   iii) a photoinitiator; and        -   iv) a polymerization inhibitor;    -    wherein the reservoir contains a transparent bottom, and        wherein actinic radiation is capable of shining through the        transparent bottom; and    -   b) providing a carrier plate comprising a substrate and a cured        layer of photopolymer on said substrate such that the cured        layer of photopolymer acts as the floor layer of the        flexographic printing plate, wherein the carrier plate provides        a surface on which the printing plate is formed, wherein the        reservoir is below the carrier plate;    -   c) bringing the cured layer of photopolymer on the carrier plate        into contact with the photocurable composition in the reservoir;    -   d) selectively providing actinic radiation beneath the        transparent bottom of the reservoir in a pattern that        corresponds to raised relief structures to be formed on the        carrier plate, wherein the radiation crosslinks and cures areas        of the photocurable composition in the reservoir;    -   e) moving the carrier plate away from the reservoir while the        actinic radiation continuously crosslinks and cures the        photocurable composition to form a flexographic printing plate,        wherein raised features of the flexographic printing plate are        formed on the cured layer of photopolymer on the carrier plate        simultaneously as the carrier plate is withdrawn from the        reservoir.

Further to that end, in another embodiment, a continuous liquidinterphase method of making a flexographic printing plate is provided,comprising:

-   -   a) providing a photocurable composition in a reservoir of a        three dimensional printer, wherein the photocurable composition        comprises:        -   i) a binder resin;        -   ii) monomers; and        -   iii) a photoinitiator; and    -   b) providing a carrier plate comprising a substrate and a cured        layer of photopolymer on said substrate such that the cured        layer of photopolymer acts as the floor layer of the        flexographic printing plate, wherein the carrier plate provides        a surface on which the flexographic printing plate is formed;        and    -   c) using the three dimensional printer to print photocurable        composition onto the cured layer of photopolymer while exposing        the printed photocurable composition to actinic radiation to        cure such printed photocurable composition to form raised relief        features on the cured layer of photopolymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an improved method of makingflexographic printing plates using a continuous liquid interphase 3Dmethod, including without limitation continuous three dimensionalprinting.

By using a continuous liquid interphase printing method, a flexographicprinting plate can be fabricated continuously, rather than layer bylayer, which substantially reduces or eliminates the presence ofcleavage lines present from typical additive methods. This also resultsin significant reduction in time needed to produce a flexographicprinting plate.

The carrier plate comprises a substrate layer and a layer of curedphotopolymer on the substrate layer. The substrate layer compriseseither a flexible sheet of plastic material or a sheet of metal. Thelayer of cured photopolymer comprises any photopolymer useful in thefabrication of flexographic printing plates.

By using a precured layer of photopolymer as the floor layer substantialtime is saved in the creation of the printing plate. Having the precuredfloor in place reduces the amount of photopolymer that must be appliedvia the liquid interphase printing method. Having the precured flooralso allows for the ability to utilize either a photopolymer that is thesame as the photopolymer used to create the relief or differenttherefrom. Using a photopolymer for the floor that has differentphysical properties from the photopolymer used to create the reliefallows for a better fine tuning of the overall performancecharacteristics of the resulting flexographic printing plate.

As described herein, the purpose of the present invention is to providean improved method of making a flexographic printing plate using acontinuous liquid interphase method.

In one embodiment, the continuous process begins with a reservoir ofliquid photopolymer, in which the bottom of the reservoir is transparentto actinic radiation. A carrier plate, which is the plate upon which theflexographic printing plate is to be built, is initially in directcontact with the liquid photocurable composition and is subsequentlywithdrawn from the from the reservoir as polymerization occurs. Anactinic radiation source shines through the transparent bottom of thereservoir and selectively crosslinks and cures the liquid photocurableresin to form a flexographic printing plate. As the flexographicprinting plate is selectively formed, the carrier plate is withdrawnfrom the base of reservoir. The liquid photocurable compositioncontinues to be polymerized at the base of the reservoir until theflexographic printing plate is fully formed.

The base of the reservoir contains a surface that is semipermeable tothe polymerization inhibitor which is present in the photocurablecomposition. There is a gradient formed within the reservoir in whichthe solid material created by the cross-linking of the photocurablecomposition and the non-polymerized material overlap at least partially.There is not a sharp interface defined by the amount of polymerizationthat has occurred and the amount of uncured photocurable compositionpresent in the reservoir.

The semipermeable surface comprises a fluoropolymer, a rigidgas-permeable polymer, porous glass, or combination thereof. Because thepolymerization inhibitor passes onto this surface, the build-up of curedpolymer is prevented at the base of the reservoir, thereby promoting thecured photocurable composition to polymerize and cure on the carrierplate or onto the previously cured photocurable composition that hasbeen selectively crosslinked and cured to form a flexographic printingplate. The reservoir can be filled with additional photocurablecomposition as needed.

The reservoir is essentially fixed or stationary, while the carrierplate moves away from the reservoir during production of the printingplate. Essentially fixed or stationary means only minor motion shouldoccur that does not disrupt the continuous production of polymerizedphotocurable composition during the production of the flexographicprinting plate. If disrupted, the polymerization may continue, althougha cleavage line may form. Additionally, predetermined cleavage lines maybe formed at locations deemed desirable before further continuousformation proceeds. The flexographic printing plates can be producedeither parallel or perpendicular relative to the surface of thephotocurable composition contained in the reservoir.

The source of actinic radiation is located below the reservoir andshines into the transparent bottom of the reservoir. Any conventionalsources of actinic radiation can be used for this crosslinking andcuring of the photocurable composition, including, for example, carbonarcs, mercury-vapor arcs, fluorescent lamps, electron flash units,electron beam units, LEDs and photographic flood lamps.

The formation of polymerized material on the surface of the carrierplate is continuous and the polymerized material is in constant contactwith the photocurable composition in the reservoir until the formationof the flexographic printing plate is complete.

To that end, in one embodiment, a continuous liquid interphase method ofmaking a flexographic printing plate is provided, comprising:

-   -   a) providing a photocurable composition in a reservoir, wherein        the photocurable composition comprises:        -   i) a binder resin;        -   ii) monomers;        -   iii) a photoinitiator; and        -   iv) a polymerization inhibitor;    -    wherein the reservoir contains a transparent bottom, and        wherein actinic radiation is capable of shining through the        transparent bottom; and    -   b) providing a carrier plate comprising a substrate and a cured        layer of photopolymer on said substrate such that the cured        layer of photopolymer acts as the floor layer of the        flexographic printing plate, wherein the carrier plate provides        a surface on which the printing plate is formed, wherein the        reservoir is below the carrier plate;    -   c) bringing the cured layer of photopolymer on the carrier plate        into contact with the photocurable composition in the reservoir;    -   d) selectively providing actinic radiation beneath the        transparent bottom of the reservoir in a pattern that        corresponds to raised relief structures to be formed on the        carrier plate, wherein the radiation crosslinks and cures areas        of the photocurable composition in the reservoir;    -   e) moving the carrier plate away from the reservoir while the        actinic radiation continuously crosslinks and cures the        photocurable composition to form a flexographic printing plate,        wherein raised features of the flexographic printing plate are        formed on the cured layer of photopolymer on the carrier plate        simultaneously as the carrier plate is withdrawn from the        reservoir.

If three dimensional printing of the photocurable layer is used, thethree dimensional printing device will generally comprise a reservoir tohold the photocurable material, an imaging system for curing thephotocurable material as it is printed, and preferably a liftingplatform for connecting to a molding tool. The imaging system canirradiate a light beam image to enable a portion of photocurablematerial to be cured as it is printed. Preferably, after the imagingsystem irradiates the light beam image, the lifting platform will drivephotocurable material to slightly drop onto the surface which is beingprinted upon. Such a suitable three dimensional printing device isdescribed in US Patent Application No. 2016/030266A1, the teachings ofwhich are incorporated herein by reference in their entirety.

Further to that end, in another embodiment, a continuous liquidinterphase method of making a flexographic printing plate is provided,comprising:

-   -   a) providing a photocurable composition in a reservoir of a        three dimensional printer, wherein the photocurable composition        comprises:        -   i) a binder resin;        -   ii) monomers; and        -   iii) a photoinitiator; and    -   b) providing a carrier plate comprising a substrate and a cured        layer of photopolymer on said substrate such that the cured        layer of photopolymer acts as the floor layer of the        flexographic printing plate, wherein the carrier plate provides        a surface on which the flexographic printing plate is formed;        and    -   c) using the three dimensional printer to print photocurable        composition onto the cured layer of photopolymer while exposing        the printed photocurable composition to actinic radiation to        cure such printed photocurable composition to form raised relief        features on the cured layer of photopolymer.

A detackification step may be used if necessary and can involve the useof a germicidal unit (light finisher) to ensure a totally tack-freeplate surface. This step is not required for all plates, as certainresins may be tack-free and thus printing press ready without the needfor the detackification step.

The photocurable composition generally comprises one or more resins,binders and/or plasticizers in combination with one or morephoto-initiators and one or more polymerization inhibitors.

Binder resins suitable for use in the present invention areaddition-polymerizable ethylenically unsaturated compounds. Thephotocurable composition may contain a single resin or a mixture ofresins. The photocurable composition also contains monomers that aretypically reactive monomers especially acrylates and methacrylates. Suchreactive monomers include, but are not limited to, trimethylolpropanetriacrylate, hexanediol diacrylate, 1,3-butylene glycol diacrylate,diethylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentylglycol diacrylate, polyethylene glycol-200 diacrylate, tetraethyleneglycol diacrylate, triethylene glycol diacrylate, pentaerythritoltetraacrylate, tripropylene glycol diacrylate, ethoxylated bisphenol-Adiacrylate, trimethylolpropane triacrylate, di-imethylolpropanetetraacrylate, triacrylate of tris(hydroxyethyl)isocyanurate,dipentaerythritol hydroxypentaacrylate, pentaerythritol triacrylate,ethoxylated trimethylolpropane triacrylate, triethylene glycoldimethacrylate, ethylene glycol dimethacrylate, tetraethylene glycoldimethacrylate, polyethylene glycol-200 dimethacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycol-600dimethacrylate, 1,3-butylene glycol dimethacrylate, ethoxylatedbisphenol-A dimethacrylate, trimethylolpropane trimethacrylate,diethylene glycol dimethacrylate. 1,4-butanediol diacrylate, diethyleneglycol dimethacrylate, pentaerythritol tetramethacrylate, glycerindimethacrylate, trimethylolpropane dimethacrylate, pentaerythritoltrimethacrylate, pentaerythritol dimethacrylate, pentaerythritoldiacrylate, urethanemethacrylate or acrylate oligomers and the likewhich can be added to the photopolymerizable composition to modify thecured product. Monoacrylates including, for example, cyclohexylacrylate, isobornyl acrylate, lauryl acrylate and tetrahydrofurfurylacrylate and the corresponding methacrylates are also usable in thepractice of the invention. It is generally preferred that the one ormore resins be present in at least an amount of 20% by weight of thephotocurable composition.

Binder resins such as styrenic block copolymers are additionally usablein the compositions of the invention. Suitable binder materials includenatural or synthetic polymers of conjugated diolefin hydrocarbons,including 1,2-polybutadiene, 1,4-polybutadiene, butadiene/acrylonitrile,butadiene/styrene, thermoplastic-elastomeric block copolymers e.g.,styrene-butadiene-styrene block copolymer, styrene-isoprene-styreneblock copolymer, etc., and copolymers. It is generally preferred thatthe styrenic block copolymers be present in at least an amount of 5% byweight of the photocurable composition.

The photocurable composition also optionally contains a compatibleplasticizer. Suitable plasticizers include, but are not limited to,dialkyl phthalates, alkyl phosphates, polyethylene glycol, polyethyleneglycol esters, polyethylene glycol ethers, polybutadiene, polybutadienestyrene copolymers, hydrogenated, heavy naphthenic oils, hydrogenated,heavy paraffinic oils, and polyisoprenes. Other useful plasticizersinclude oleic acid, lauric acid, etc. If used, the plasticizer isgenerally present in an amount of at least 5% by weight, based on weightof total solids of the photocurable composition.

Photoinitiators for use in the photocurable composition include benzoinalkyl ethers, such as benzoin methyl ether, benzoin ethyl ether, benzoinisopropyl ether and benzoin isobutyl ether. Another class ofphotoinitiators are the dialkoxyacetophenones such as2,2-dimethoxy-2-phenylacetophenone and2,2-diethoxy-2-phenylacetophenone. Still another class ofphotoinitiators are the aldehyde and ketone carbonyl compounds having atleast one aromatic nucleus attached directly to the carboxyl group.These photoinitiators include, but are not limited to, benzophenone,acetophenone, o-methoxybenzophenone, acenaphthenequinone, methyl ethylketone, valerophenone, hexanophenone, alpha-phenylbutyrophenone,p-morpholinopropiophenone, dibenzosuberone, 4-morpholinobenzophenone,4′-morpholinodeoxybenzoin, p-diacetylbenzene, 4-aminobenzophenone,4′-methoxyacetopherione, benzaldehyde, alpha-tetralone,9-acetylphenarithrene, 2-acetylphenanthrene, 10-thioxanthenone,3-acetylphenanthrene, 3-acetylindone, 9-fluorenone, 1-indanone,1,3,5-triacetylbenzene, thioxanthen-9-one, xanthene-9-one,7-H-benz[de]-anthracene-7-one, 1-naphthaldehyde,4,4.degree.-bis(dimethylamino)-benzophenone, fluorene-9-one,1′-acetonaphthone, 2′-acetonaphthone, 2,3-butanedione, acetonaphthene,benz[a]anthracene 7.12 dione, etc. Phosphines such as triphenylphosphineand tri-otolylphosphine can also be used as photoinitiators. Both freeradical and cationic types of photopolymerization initiators may beused. It is generally preferred that the photoinitiators be present inat least an amount of 0.1% by weight of the photocurable composition.

Polymerization inhibitors for use in the photocurable compositioninclude, for example, p-methoxyphenol, hydroquinone, and alkyl andaryl-substituted hydroquinones and quinones, tert-butyl catechol,pyrogallol, copper resinate, naphthalamines, beta-naphthol, cuprouschloride, 2,6-di-tert-butyl-p-cresol, butylated hydroxytoluene (BHT),oxalic acid, phenothiazine, pyridine, nitrobenzene and dinitrobenzene,p-toluquinone and chloranil. While in some instances it may be desirableto include a polymerization inhibitor such as BHT or similarpolymerization inhibitors in the photopolymerizable composition, caremust be taken to use BHT and other similar polymerization inhibitorsonly in an amount and with a combination of other additives such that itdoes not compromise the imaging properties of the photopolymerizableresin. The polymerization inhibitors may be used in the photocurablecomposition in an amount of about 0.05 to about 5% by weight.Polymerization inhibitors may or may not be used, but they arepreferably used in the embodiment of the invention that involvescontinuous curing in the reservoir of photocurable composition.

Various dyes and/or colorants may also optionally be used in thepractice of the invention although the inclusion of a dye and/orcolorant is not necessary to attain the benefits of the presentinvention. Suitable colorants are designated “window dyes” which do notabsorb actinic radiation in the region of the spectrum that theinitiator present in the composition is activatable. The colorantsinclude, for example, CI 109 Red dye, Methylene Violet (CI Basic Violet5), “Luxol.” Fast Blue MBSN (CI Solvent Blue 38), “Pontacyl” Wool BlueBL (CI Acid Blue 59 or CI 50315), “Pontacyl” Wool Blue GL (CI Acid Blue102 or CI 50320), Victoria Pure Blue BO (CI Basic Blue 7 or CI 42595),Rhodamine 3 GO (CI Basic Red 4), Rhodamine 6 GDN (CI Basic Red I or CI45160), 1,1′-diethyl-2,2′-cyanine iodide, Fuchsine dye (CI 42510),Calcocid Green S (CI 44090), Anthraquinone Blue 2 GA (CI Acid Blue 58),Solvaperm Red BB (Solvent Red 195), etc.

Other additives including antiozonants, fillers or reinforcing agents,UV absorbers, etc. may also be included in the photocurable composition,depending on the final properties desired. Such additives are generallywell known in the art. However, care must be taken to ensure that theuse of these other additives do not compromise the crosslinkingproperties of the photocurable composition.

Suitable fillers and/or reinforcing agents include immiscible, polymericor nonpolymeric organic or inorganic fillers or reinforcing agents whichare essentially transparent at the wavelengths used for curing thephotocurable composition and which do not scatter actinic radiation,e.g., polystyrene, the organophilic silicas, bentonites, silica,powdered glass, colloidal carbon, as well as various types of dyes andpigments. Such materials are used in amounts varying with the desiredproperties of the elastomeric compositions. The fillers are useful inimproving the strength of the elastomeric layer, reducing tack and, inaddition, as coloring agents.

By using continuous liquid interphase 3D methods to produce aflexographic printing plate, the resulting flexographic printing platehas a Shore A hardness of between 25 and 95, preferably about 45 andabout 70, more preferably between about 50 and about 65. The resultingflexographic printing plate has elastomeric elongations preferablygreater than 50%. The thickness of the flexographic printing plate isbetween about 0.030 and about 0.250 inches. By using a floor of procuredphotopolymer, the physical properties of the floor can be different fromthe physical properties of the relief features.

A plurality of printing dots, with desirable characteristics forprinting, can be produced on the surface of the flexographic printingplate using a continuous liquid interphase production method. Theplanarity of the top of a dot can be measured as the radius of curvatureacross the top surface of the dot, r_(e). It is noted that a rounded dotsurface is not ideal from a printing perspective because the size of thecontact patch between the print surface and the dot varies exponentiallywith impression force. Therefore, the top of the dot preferably has aplanarity where the radius of curvature of the dot top is greater thanthe thickness of the crosslinked photocurable composition layer, morepreferably at least twice the thickness of the layer, and mostpreferably more than three times the total thickness of the crosslinkedphotocurable composition layer.

Another desirable printing dot characteristic is edge sharpness. Edgesharpness relates to the presence of a well-defined boundary between theprinting dot top and the shoulder and it is generally preferred that thedot edges be sharp and defined. These well-defined dot edges betterseparate the “printing” portion from the “support” portion of the dot,allowing for a more consistent contact area between the dot and thesubstrate during printing.

Edge sharpness can be defined as the ratio of r_(e), the radius ofcurvature (at the intersection of the shoulder and the top of the dot)to p, the width of the dot's top or printing surface. For a trulyround-tipped dot, it is difficult to define the exact printing surfacebecause there is not really an edge in the commonly understood sense,and the ratio of r_(e):p can approach 50%. In contrast, a sharp-edgeddot would have a very small value of r_(e), and r_(e):p would approachzero. In practice, an r_(e):p of less than 5% is preferred, with anr_(e):p of less than 2% being most preferred.

The continuous liquid interphase method of printing, including threedimensional printing, allows for the tailoring of the shape of theprinting dots produced such that relief structures such as dots can beprinted in the precise shape determined to provide the optimum printingcharacteristics.

In addition to creating preferred printing dot characteristics on theflexographic printing plate, by using a continuous liquid interphaseproduction method, there is little to no waste. All unused photopolymercomposition remains in the reservoir and can be used in the productionof further flexographic printing plates or stored for subsequent use.The continuous liquid interphase method for producing a flexographicprinting plate is also much less time consuming than traditional 3Dprinting methods, which involve a repetitive layering process thattypically takes hours or up to days to complete. A flexographic printingplate can be produced in a matter of minutes using this continuousliquid interphase method.

Finally, it should also be understood that the following claims areintended to cover all of the generic and specific features of theinvention described herein and all statements of the scope of theinvention that as a matter of language might fall there between.

What is claimed is:
 1. A method of making a flexographic printing plateis provided, said method comprising: a) providing a photocurablecomposition in a reservoir, wherein the photocurable compositioncomprises: i) a binder resin; ii) monomers; iii) a photoinitiator; andiv) a polymerization inhibitor;  wherein the reservoir contains atransparent bottom, and wherein actinic radiation is capable of shiningthrough the transparent bottom; and b) providing a carrier platecomprising a substrate and a cured layer of photopolymer on saidsubstrate such that the cured layer of photopolymer acts as the floorlayer of the flexographic printing plate, wherein the carrier plateprovides a surface on which the printing plate is formed, wherein thereservoir is below the carrier plate; c) bringing the cured layer ofphotopolymer on the carrier plate into contact with the photocurablecomposition in the reservoir; d) selectively providing actinic radiationbeneath the transparent bottom of the reservoir in a pattern thatcorresponds to raised relief structures to be formed on the carrierplate, wherein the radiation crosslinks and cures areas of thephotocurable composition in the reservoir; e) moving the carrier plateaway from the reservoir while the actinic radiation continuouslycrosslinks and cures the photocurable composition to form a flexographicprinting plate, wherein raised features of the flexographic printingplate are formed on the cured layer of photopolymer on the carrier platesimultaneously as the carrier plate is withdrawn from the reservoir. 2.The method according to claim 1, wherein the photocurable compositionfurther comprises a filler.
 3. The method according to claim 1, whereinthe flexographic printing plate comprises a plurality of printing dots.4. The method according to claim 3, wherein the printing dots have aplanarity where the radius of curvature of the dot top is greater thanthe thickness of the crosslinked and cured photocurable composition. 5.The method according to claim 4, wherein the printing dots have aplanarity where the radius of curvature of the dot top is at least twicethe thickness of the crosslinked and cured photocurable composition. 6.The method according to claim 5, wherein the printing dots have aplanarity where the radius of curvature of the dot top is more thanthree times the total thickness of the crosslinked and curedphotocurable composition.
 7. The method according to claim 1, whereinthe printing dots have an r_(e):p of less than 5%.
 8. The methodaccording to claim 7, wherein the printing dots have an r_(e):p of lessthan 2%.
 9. The method according to claim 1, wherein the flexographicprinting plate has elastomeric elongation greater than 50%.
 10. Themethod according to claim 1, wherein the physical properties of thephotocurable composition, when cured, are different from the physicalproperties of the cured layer of photopolymer.
 11. The method accordingto claim 1, wherein the thickness of the flexographic printing plate isbetween about 0.030 and about 0.250 inches.
 12. The method according toclaim 1, wherein the polymerization inhibitor is selected from the groupconsisting of p-methoxyphenol, hydroquinone, and alkyl andaryl-substituted hydroquinones and quinones, tert-butyl catechol,pyrogallol, copper resinate, naphthalamines, beta-naphthol, cuprouschloride, 2,6-di-tert-butyl-p-cresol, butylated hydroxytoluene (BHT),oxalic acid, phenothiazine, pyridine, nitrobenzene and dinitrobenzene,p-toluquinone, chloranil and combinations thereof.
 13. A method ofmaking a flexographic printing plate, said method comprising: a)providing a photocurable composition in a reservoir of a threedimensional printer, wherein the photocurable composition comprises: i)a binder resin; ii) monomers; and iii) a photoinitiator; and b)providing a carrier plate comprising a substrate and a cured layer ofphotopolymer on said substrate such that the cured layer of photopolymeracts as the floor layer of the flexographic printing plate, wherein thecarrier plate provides a surface on which the flexographic printingplate is formed; and c) using the three dimensional printer to printphotocurable composition onto the cured layer of photopolymer whileexposing the printed photocurable composition to actinic radiation tocure such printed photocurable composition to form raised relieffeatures on the cured layer of photopolymer.
 14. The method according toclaim 13, wherein the flexographic printing plate comprises a pluralityof printing dots.
 15. The method according to claim 14, wherein theprinting dots have a planarity where the radius of curvature of the dottop is greater than the thickness of the crosslinked and curedphotocurable composition.
 16. The method according to claim 15, whereinthe printing dots have a planarity where the radius of curvature of thedot top is at least twice the thickness of the crosslinked and curedphotocurable composition.
 17. The method according to claim 16, whereinthe printing dots have a planarity where the radius of curvature of thedot top is more than three times the total thickness of the crosslinkedand cured photocurable composition.
 18. The method according to claim13, wherein the printing dots have an r_(e):p of less than 5%.
 19. Themethod according to claim 18, wherein the printing dots have an r_(e):pof less than 2%.
 20. The method according to claim 13, wherein theflexographic printing plate has elastomeric elongation greater than 50%.21. The method according to claim 13, wherein the physical properties ofthe photocurable composition, when cured, are different from thephysical properties of the cured layer of photopolymer.
 22. The methodaccording to claim 13, wherein the thickness of the flexographicprinting plate is between about 0.030 and about 0.250 inches.